1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method for correcting a color shade of a color mixture and a storage medium in which an image processing program has been stored.
2. Description of the Related Art
In recent years, performance of an electrophotographic apparatus has been improved and an apparatus which can realize picture quality similar to that of a printer has been put into practical use. However, such a problem that a fluctuation amount of color is larger than that of the printer because of instability that is peculiar to the electrophotography still remains. To solve such a problem, a calibration technique for forming an LUT (Look Up Table) for one-dimensional tone correction corresponding to each toner of cyan, magenta, yellow, and black has been installed in the electrophotographic apparatus in the related art. The LUT is a table showing output data corresponding to input data partitioned by a specific interval and can express non-linear characteristics which cannot be expressed by an arithmetic operational expression. According to the LUT for 1-dimensional tone correction, each input signal value of cyan, magenta, yellow, and black is converted into an output signal value which can be expressed on the printer side and an image is formed onto paper by using toner of an amount corresponding to the output signal value. As a method of forming the LUT for 1-dimensional tone correction, first, a chart constructed by data of a different tone corresponding to each toner of cyan, magenta, yellow, and black is prepared and output by the printer. Subsequently, a value is read out from the chart output by the printer by using a scanner, a colorimetry apparatus, or the like. By comparing the read value with target data which has previously been stored, the 1-dimensional LUT for correction is formed.
However, even if the tone characteristics of a monochrome are combined by the 1-dimensional LUT, it is difficult to guarantee a color reproduction of the color mixture in which the cyan, magenta, and yellow toner of red, green, blue, black, and the like have been mixed. Therefore, such a technique that an attention is paid to a destination profile among ICC (International Color Consortium) profiles and the color reproduction of the color mixture is corrected by modifying the destination profile has been proposed (for example, the Official Gazette of Japanese Patent Application Laid-Open No. 2006-165864).
First, the chart formed by the color mixture is output by the printer and the output chart is measured by the scanner or colorimetry apparatus. A difference is formed by using a colorimetry result and a target value, a 3-dimensional LUT (destination profile) for converting a device independent color space (L*a*b*) which the ICC profiles have into a device dependent color space (CMYK) is updated in accordance with the formed difference, and a color shade of the color mixture can be corrected. L*a*b* is one of the color spaces which do not depend on the device, L* denotes a luminance, and a*b* denotes a hue and a saturation.
However, according to the technique for correcting the color mixture in the related art, since a process for correcting the destination profile among the ICC profiles is executed, there is such a problem that when the user uses a plurality of destination profiles, the correcting process has to be executed to all of the profiles. In addition, the color shade of the color mixture cannot be corrected to an LUT which does not use the device independent color space such as a device link profile or the like or to a process which does not use a profile such as a copy or the like instead of using the destination profile.
Further, although a method of correcting by using a multi-dimensional LUT in order to correct the color shade of the color mixture is considered, when it is intended to correct the multi-dimensional LUT, such a problem that the number of data for correction increases occurs. For example, in the case of forming correction data by using the chart of level 32 of each color of CMYK by using the 1-dimensional LUT, 128 (=32 levels×4 colors) data are necessary. Likewise, in the case where the same method as that of the 1-dimensional LUT is used to a 4-dimensional LUT, 1048576 (=32 levels (cyan)×32 levels (magenta)×32 levels (yellow)×32 levels (black)) are necessary in order to obtain a similar precision. There is, consequently, such a problem that an amount of data of the charts to form the correction data becomes very large.